1. Field of the Invention
The present invention relates generally to transgenic maize plants which are resistant to the herbicide glyphosate. More specifically, it relates to the maize transformation events GA21, GG25, FI117 and GJ11.
2. Description of the Related Art
Chemical weed control is a powerful tool of our technological age. Long known as one of the most arduous of agricultural operations, weed killing has taken on an entirely new aspect as chemical after chemical is added to the arsenal of herbicides. The U.S. has led the world both in production and use of herbicides and as a result yields of maize, soybeans, cotton, sugar beets, and many other crops have increased since 1945, in some cases 100% or more. Thus while use of fertilizers and new high-yielding crop varieties have contributed greatly to the "green revolution" chemical weed control has been at the forefront in technological achievement.
A particularly useful type of herbicide is one having a broad spectrum of herbicidal activity. Use of such herbicides obviates the need for application of multiple herbicides. The problem with such herbicides is that they typically have a deleterious effect on any crops which are exposed to the herbicide. One way to overcome this is to produce transformed crop plants with genes which confer resistance to certain broad spectrum herbicides.
Recent advances in genetic engineering have provided the requisite tools to transform plants to contain foreign genes. Plants may, therefore, be produced which have unique characteristics of agronomic importance. Certainly, weed control via herbicide tolerance is one such advantageous trait which is highly cost effective and environmentally compatible. Herbicide-tolerant plants may reduce the need for tillage to control weeds, thereby effectively reducing soil erosion. Further, herbicide resistant plants can reduce the number of different herbicides applied in the field.
One herbicide which is the subject of much investigation in this regard is N-phosphonomethyl-glycine, commonly referred to as glyphosate. Glyphosate inhibits the shikimic acid pathway which leads to the biosynthesis of aromatic compounds including amino acids and vitamins. Specifically, glyphosate inhibits the conversion of phosphoenolpyruvic acid and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvyl-3-phosphoshikimic acid synthase (EPSP synthase or EPSPS).
It has been shown that glyphosate tolerant plants can be produced by introducing, into the genome of the plant, the capacity to produce a higher level of EPSP synthase which enzyme is preferably glyphosate tolerant (Shah et al., 1986). The introduction into plants of glyphosate degradation gene(s) can provide a means of conferring glyphosate tolerance to plants and/or to augment the tolerance of transgenic plants already expressing a glyphosate tolerant EPSP synthase depending upon the physiological effects of the degradation products.
Glyphosate metabolism (degradation) has been examined in a wide variety of plants and little degradation has been reported in most of those studies. In those instances where degradation has been reported, the initial breakdown product is usually aminomethylphosphonate (AMPA) (Coupland, 1985; Marshall et al., 1987). In these instances, it is not clear if glyphosate is metabolized by the plant or by the contaminating microbes on the leaf surface to which glyphosate was applied. AMPA has been reported to be much less phytotoxic than glyphosate for most plant species (Franz, 1985) but not for all plant species (Maier, 1983; Tanaka et al., 1986). Glyphosate degradation in soils is much more extensive and rapid (Torstensson, 1985). The principal breakdown product identified is AMPA (Rueppel et al., 1977; Nomura and Hilton, 1977); a phosphonate that can be metabolized by a wide variety of microorganisms (Zeleznick et al., 1963; Mastalerz et al., 1965; Cook et al., 1978; Daughton et al., 1979a; 1979b; 1979c; Wackett et al., 1987a). A number of pure cultures of bacteria have been identified that degrade glyphosate by one of the two known routes (Schowanek and Verstraete, 1990; Weidhase et al., 1990; Liu et al., 1991). A route involving a "C--P lyase" that degrades glyphosate to sarcosine and inorganic orthophosphate (Pi) has been reported for a Pseudomonas sp. (Shinabarger and Braymer, 1986; Kishore and Jacob, 1987) and an Arthrobacter sp. (Pipke et al., 1987b). Pure cultures capable of degrading glyphosate to AMPA have been reported for a Flavobacterium sp. (Balthazor and Hallas, 1986), for a Pseudomonas sp. (Jacob et al., 1988) and for Arthrobacter atrocyaneus (Pipke and Amrhein, 1988). In addition, a large number of isolates that convert glyphosate to AMPA have been identified from industrial activated sludges that treat glyphosate wastes (Hallas et al., 1988). However, the number and nature of bacterial genes responsible for these degradations have not been heretofore determined nor have the gene(s) been isolated.
The development of plants resistant to the herbicidal compound glyphosate has been a goal in the engineering of many plant species (U.S. Pat. No. 4,769,061). The development of glyphosate resistant tobacco plants was reported by Comai et al., (1985). Herbicide resistance was conferred on plants by expression of an aroA gene derived from Salmonella typhimurium encoding a glyphosate resistant form of the enzyme EPSP synthase. In addition, glyphosate resistant soybeans were produced (Monsanto, APHIS petition 93-258-01p). Methods for production of glyphosate resistant corn plants also have been described (WO 95/06128; U.S. Pat. No. 5,554,798). Similarly, a glyphosate oxidoreductase gene has been described for use in conferring glyphosate resistance (U.S. Pat. No. 5,463,175).
The ultimate goal in producing transgenic glyphosate resistant maize plants is to provide plants which may be treated with glyphosate at a level sufficient for killing weeds, without a deleterious effect on yield or fertility. In this respect, the prior art has failed. There is, therefore, a great need in agriculture for maize plants which can be directly sprayed in the field with glyphosate, thereby killing weeds, but otherwise not producing a deleterious effect on the crop itself.